Understanding Decompression Sickness

[RXTdiver]

When you breathe in air as you go deeper, nitrogen will dissolve and enter your body tissue. As you ascend, the nitrogen will leave your body tissue which you expel as you exhale. In the PADI online course and in some videos on YouTube, there is talk about excess nitrogen.

1. When does this excess nitrogen form, while descending or ascending?

2. How does this excess nitrogen form?

 

[tbone1004]

At the highest level this is is fundamental to Daltons Law of Partial Pressure. At 33ft of salt water you are exposed to 2ata of pressure of which ~80% is made up of nitrogen. Since this gas is at twice the pressure of sea level, your tissues can hold twice the mass of gas even though it is at the same volume per Boyles Law. If we saw that a tissue holds 8 units of nitrogen at sea level out of 10 units total and we take that tissue to 2 atmospheres with the same mixture of air then the gas that you are inspiring is the same volume but now has 20 units of total gas, 16 of which are nitrogen. Over time the 16 units are pushing against the cell walls to equalize with the 8 units that are on the other side and will eventually have 16 units on both sides *you are constantly inspiring gas so any units that cross that boundary are replaced with fresh ones*.
You now start ascending and come back to the surface where your cells contain 16 units of nitrogen but the gas you are breathing only contains 8 units of nitrogen because you are at the surface, the difference is the excess nitrogen in this case but would apply to any/all inert gases that can diffuse across cell walls and are not metabolized

 

[OTF]

Good questions.

Air is 21% oxygen and 78% nitrogen. So mostly nitrogen. Also oxygen is metabolized by the body which makes it less likely to accumulate or form harmful bubbles which is why you only hear about the N2.

Anyway, the most intuitive (though simplified) analogy would be a bottle of soda or a seltzer maker. Although soda is CO2 dissolved in water, and decompression science is nitrogen dissolved in people. It works similarly.

So you start out with some regular water, it has a normal tiny amount of dissolved CO2 in equilibrium with the atmosphere around it. (This is equivalent to a diver before descending) Then you add CO2 under lots of pressure so it dissolves into the water making seltzer. (This is equivalent to the diver doing a deep dive that dissolves lots of nitrogen into the their body tissues.) If the soda bottle remains pressurized CO2 stays in solution with no bubbles. If you then release the pressure by opening the bottle the dissolved CO2 rapidly comes out of solution forming fizzy gas bubbles. (This would be equivalent to the diver making a very rapid ascent. The soda bubbles are equivalent to the nitrogen bubbles that cause decompression sickness.) However, if you were to very very very slowly open the top for a controlled release of pressure in the soda bottle, then the CO2 could slowly dissolve out of the seltzer and actually wouldn't form visible bubbles at all. (This is the equivalent to the slow ascent (or decompression stops) that prevent decompression sickness in diving.)

So to answer your question directly, the nitrogen is being absorbed into your pressurized body during the descent and bottom phase of your dive from the air you are breathing, and released as you ascend and in the hours after your dive. It's actually a little bit more complicated than that, but that's why we have tables and computers and dive planning software.

There is no other magical source of "excess" nitrogen, only deep dives and long bottom times that mean lots of nitrogen is getting absorbed from the breathing gas.

 

[kaylee_ann]

The way I understand it is in terms of partial pressure. (ppN2= N2%*pressure)
Say you dive to 100 feet, so 3 atm of pressure using nitrox 32. 0.68*3= 2.04%

Naturally, the deeper you go, the higher the atmospheric pressure. For example, a shallower depth of 66 feet would give a ppN2 of 1.36%. So, the inspired inert gas pressure increases as you descend.

Question 1- it happens when descending
Question 2- by the inspired inert gas pressure increasing as ambient pressure increases

 

[davehicks]

TLDr; It is all about bubbles. Nitrogen gas will be absorbed in your blood the longer and deeper you dive. Ascending rapidly, or exercising vigorously can cause the nitrogen to form bubbles. If the bubbles get big enough or numerous enough they can block blood flow, causing injuries as parts of your body or brain start to die off.

So ascend slowly. Do long safety stops to allow the nitrogen and bubbles to be filtered out by your lungs. Stay well hydrated so your veins stay open to flow blood smoothly. Dont ride the red line of your NDL, and take long surface intervals.

 

[Alekseolsen]

The way I understand it is in terms of partial pressure. (ppN2= N2%*pressure)
Say you dive to 100 feet, so 3 atm of pressure using nitrox 32. 0.68*3= 2.04%

Naturally, the deeper you go, the higher the atmospheric pressure. For example, a shallower depth of 66 feet would give a ppN2 of 1.36%. So, the inspired inert gas pressure increases as you descend.

Question 1- it happens when descending
Question 2- by the inspired inert gas pressure increasing as ambient pressure increases

100 feet ~ 30 meters = 3 + 1 (surface) atm
0,68 ×4 = 2.72 atm ppN2

 

[kaylee_ann]

100 feet ~ 30 meters = 3 + 1 (surface) atm
0,68 ×4 = 2.72 atm ppN2

OH I never saw it didn’t know it was calculated with absolute. Maybe I looked at it wrong or at the wrong source. Thanks for the correction!

 

[Alekseolsen]

S

OH I never saw it didn’t know it was calculated with absolute. Maybe I looked at it wrong or at the wrong source. Thanks for the correction!

Mall error to make

If you think about it, we do have 0.79 ata ppN2 also at the surface, and double at ten meters depth.
Doing that mathematically if surface =0 atm gets difficult

 

[kaylee_ann]

S

Mall error to make

If you think about it, we do have 0.79 ata ppN2 also at the surface, and double at ten meters depth.
Doing that mathematically if surface =0 atm gets difficult

Yeah cause it would say you’re breathing 0% N2 LOL. That would most definitely be wrong.
ETA- god I’m glad I learned this now. Imagine finding the ppO2 not using absolute, and it looks safe. Turns out, it’s actually toxic. Or above where you’d want to keep your max O2 partial pressure

 

[tbone1004]

You can also visualize this in normal world with condensation on a glass of water or morning dew. When the air is warm it can hold a higher quantity of moisture but as it cools it gets knocked out in the form of condensation as a surplus. Can think of it like warming=descending, cooling=ascending where the tissues can hold more inert gas when under pressure.